I think what he was eluding to the Beacon frames being sent at higher data rates. So if there are clients that are closer to AP3 but still connecting to AP1, then increasing that threshold might make it so that they are not seeing AP1 beacon frames, so they won't connect to it. Most likely though this won't actually do that as those data rates can be heard at like -85. As long as you have it set to 12Mbps, then you should be good (to cut off 802.11b, unless you need that enabled? If not kill it, increasing airtime for clients).
Clients are the ones that will determine who they connect to, 100% of the time. Things like client balancing will initially 'refuse' to allow the client to connect, but if it is persistent enough, it will. That refusal process can actually cause issues with this like fast roaming or voice/video sensitive applications, so usually not best practice to use that feature.
Check the power level of AP1, is it unusually high?
Are all the access points using the same power levels? For both bands?
Are the clients connecting to 2.4GHz or 5GHz?
You will usually want the 2.4GHz radio power level to be about 6db lower than the power level of 5GHz, resulting in about the same cell size foot print.